In bandlimited communication systems, various techniques have been employed to operate within the restrictions imposed by the particular type of transmission system chosen. These transmission systems present difficulties at both the very low end of the frequency spectrum and at the high end of the frequency spectrum. This is due to the fact that coupling onto a transmission line, such as coax, is sometimes accomplished through the use of transformers. This results in the loss of the DC component of the signal which for some digital coding schemes is a necessity. To reconstitute the DC component requires sophisticated DC recovery circuits.
One digital transmission system for operation over a bandlimited communication channel has been described in U.S. Pat. No. 4,135,057 issued to J. W. Bayless. The Bayless patent utilizes a carrier frequency to transmit a raised cosine pulse. The raised cosine pulse is the result of driving a selected transfer function with a pulse waveform having a positive going pulse representative of a logic "high" and a negative going pulse representative of a logic "low". The transfer function is the result of cascading a transmitter transfer function, a channel transfer function and a receiver transfer function such that the overall system results in a matched filter. This matched filter reduces intersymbol interference to allow for optimum retrieval of the data.
In a wireline transmission system, such as a coax line, the noise spectral density tends to be higher for the low frequencies. In addition, the normal mode of coupling into a wireline transmission system is by a device such as a transformer which introduces further attenuation at the low frequencies. This presents disadvantages in that some transmission methods require transmission of low frequency components to maintain signal integrity. Attenuation of these low frequency components results in increased intersymbol interference and increased signal-to-noise ratio.
In view of the above problem with the prior art, there exists a need for a system that operates over a wireline channel employing transformer coupling and is operable to eliminate the need for a low frequency component to maintain signal integrity.